When we reach a whitespace after a section name, we assume that what
will follow will be a quoted subsection name.  Pass the current position
of the line being parsed to the subsection parser, so that it can
validate that subsequent characters are additional whitespace or a
single quote.

Previously we would begin parsing after the section name, looking for
the first quotation mark.  This allows invalid characters to embed
themselves between the end of the section name and the first quotation
mark, eg `[section foo "subsection"]`, which is illegal.

Opening a default config when ~/.gitconfig doesn't exist, locking it,
and attempting to write to it causes an assertion failure.

Treat non-existent global config file content as an empty string.

According to reports, libgit2 is unable to read a global configuration
file that is simply a symlink to the real configuration. Write a
(succeeding) test that shows that libgit2 _is_ correctly able to do so.

While we do verify that we are able to open the global ".gitconfig" file
in config::global::open_global, we never verify that we it is in fact
readable. Do so by writing the global configuration file and verifying
that reading from it produces the expected values.

The config::global test suite creates various different directories and
files which are being populated with pretend-global files.
Unfortunately, the tests do not clean up after themselves, which may
cause subsequent tests to fail due to cruft left behind.

Fix this by always removing created directories and their contents.

Move to the `git_error` name in the internal API for error-related
functions.

When we try to add a configuration file with `git_config_add_file_ondisk`, we
treat nonexisting files as empty. We do this by performing a stat call, ignoring
ENOENT errors. This works just fine in case the file or any of its parents
simply does not exist, but there is also the case where any of the parent
directories is not a directory, but a file. So e.g. trying to add a
configuration file "/dev/null/.gitconfig" will fail, as `errno` will be ENOTDIR
instead of ENOENT.

Catch ENOTDIR in addition to ENOENT to fix the issue. Add a test that verifies
we are able to add configuration files with such an invalid path file just fine.

config: Port config_file_fuzzer to the new in-memory backend.